Respiratory monitoring is a crucial tool for clinical diagnostics and patient safety, particularly when a subject is unconscious, such as during sleep or an induced unconscious state.
In the current diagnosis of disordered respiration, such as sleep apnea, extensive instrumentation is required. Techniques and devices used for diagnosing sleep apnea include electroencephalograms, electrocardiograms, electromyographs, pulse oximetry, ocular and body movement strain gauges, snoring recording, postural position monitoring and respiratory gas flow monitoring. In practice, only four or so of these transducers will be attached in most sleep clinics.
Nasal airflow (or pressure) transducers are generally used to provide the main signal for apnea/hypoapnea scoring, although pulse oximetry (as a measure of arterial blood-oxygen) and electroencephalography (as a measure of arousal) are used as confirmatory information. Linear displacement transducers may be used for detecting chest and abdomen expansion as a measure of respiration and sound (snoring) levels recorded. Some of these transducers have been modified for home monitoring, thus reducing costs associated with sleep clinical diagnostics, however, technical assistance at set up is still recommended based on studies comparing user or technician monitoring set up (Gaqnadoux and Pelletier-Fleury, Chest (March) 2002).
The signal derived from the transducer provides a trace showing the inhalation/exhalation cycle, and time periods of no respiratory activity. It is also useful in estimating periods of insufficient respiration, such as those characteristic of hypopnea (Hemdndez et al., Chest 2001; 119:442-450), although confirmatory evidence is generally needed for the latter, either oxygen desaturation (pulse oximeter) or else arousal (usually EEG or accelerometer) or both.
There are several causes of sleep apnea. Obstructive sleep apnea (OSA) is the more common form, characterised by the soft palette blocking the air passage. During OSA, diaphragm movement persists in respiratory effort. This does not occur with central sleep apnea, where breathing stops due to lack of respiratory muscle effort induced by lack of cerebral signal for respiratory muscle contraction. Mixed sleep apnea results when both of these effects occur.
During mouth breathing, nasal prongs give false positive apnea events, and generally the signal is poor during periods of snoring, a parameter which in itself is characteristic of sleep apnea. However, monitoring snoring as a diagnostic parameter has limited diagnostic value unless used in conjunction with other transducers (Hemdndez et al., Chest 2001; 119:442-450).
Pulse oximetry is universally used to monitor blood-oxygen saturation in patients during respiratory studies. The pulse oximeter probe is designed for clamping to a finger, and provides a measure of the arterial blood-oxygen saturation level mainly in the skin. There are several disadvantages in using oximetry for measuring hypoxia, including:    1) The skin (finger) is a low metabolizing tissue that is unlikely to reflect the oxygen saturation changes experienced by high metabolizing organs, particularly the heart and brain.    2) The finger attachment is not appropriate for use on subjects who thrash around during the study, as a consequence of continual arousal (dislodgment of the oximeter probe is one of the most common equipment failures in respiratory monitoring). Body movement leads to changes in baseline referred to as movement artefact, which can be inaccurately interpreted as desaturation events. Tightly clamping probes, which can reduce this movement artefact, tends to be uncomfortable when used for long periods.    3) The accuracy of pulse oximetry at low blood-oxygen saturation levels has been called into question by investigators (Mertzluffi and Zander, (1991); In the Oxygen Status of Arterial Blood. Ed. Zander R. and Mertzlufft F., Publ. Karger, N.Y., p 106-123).    4) The pulse-oximeter depends on pulsatile blood flow for timing in sampling. During hypoxia, pulsatile flow can be difficult to pick up in the skin due to tissue capillary-bed bypassing of the blood in the skin, via arteriolar-venular shunts, when oxygenated blood is diverted to increase critical tissue perfusion.
A positive diagnosis of sleep apnea is generally made if the instruments recording signals indicate five or more apnea events per hour (each exceeding 10 seconds) during sleep. An excess of 30 apneas per hour is usually required for a clinical diagnosis of severe sleep apnea. Many clinics also include hypoapnea scoring (insufficient respiratory rates for >2 minutes) based on respiratory airflow data and/or the number of arousal's recorded per hour.
Diagnostic scoring can be ambiguous for milder to moderate sleep-apnea conditions, with some clinicians recommending studies over two nights to reduce false positive/negative diagnosis (Le Bon et al., Chest (August) 2000). The high cost of sleep clinic studies, and the number of possible candidates to be screened, means that screening of all patients requiring this extended testing is not cost effective, and thus often not feasible.
A number of biomolecules absorb near infrared radiation at specific wavelengths via bond stretching or dipole interactions, and this property is utilised by near infrared spectroscopy (NIRS) (Dyer, (1965) pp. 22-57, Pub. Prentice-Hall Inc. New Jersey). Like oximetry, NIRS detects photons at two or more wavelengths that have been scattered through tissue. The flux densities of the emerging photons (at 750 nm and 810 nm), are proportional to the ratio of deoxyhemoglobin:oxyhemoglobin within the tissue, and can be used to calculate the physiological parameter of blood hemoglobin oxygen saturation.
NIRS methodology differs from pulse oximetry in two important respects:    (1) Pulse oximetry relies in part on visible radiation, usually at 660 nm, which limits photon penetration to several millimeters travel through tissue, whereas near infrared radiation has significantly greater penetration;    (2) NIRS is optimized to measure deoxyhemoglobin proportions, whereas pulse oximetry which is optimized to measure oxyhemoglobin levels.
The low penetration depth of the radiation used in pulse oximetry (less than 10 mm at a wavelength of ˜660 nm), means it is unsuitable for non-invasive deep tissue blood-oxygen assessment. In contrast, a device using NIR-radiation only can readily penetrate bone and assess deep tissue.